After [clean-up] crews left Florida and Alabama, closer inspection revealed tiny black tar balls, the [University of South Florida] scientists said. They also said cleanup efforts did not address layers of oil buried inches below the sand, accounting for possibly more than half of the beach contamination.

“That leads us to the conclusion that the cleanup at this point is basically superficial,” said Ping Wang, a USF beach geologist who led the research. “That maybe using these machines which are crushing the tar into small pieces may not be the smartest thing to do.”
Read the article here: http://www.tampabay.com/news/environment/bp-cleanup-efforts-missed-thousands-of-tar-balls-usf-researchers-said/1113562